1. Field of the Invention
This invention relates to solid oxide fuel cells (SOFC) and to methods for plating their interconnections.
2. Background Information
A solid oxide fuel cell is an electrochemical cell that burns a fuel to generate heat and electricity. In one embodiment, the fuel cell consists of a multilayered tube that is electrically connected to other tubes. The electrical connection between the cells can be made through a ceramic interconnection or a segment of anode (fuel electrode) material, a nickel-zirconia cermet or cobalt zirconia cermet material, which coats the interconnection as taught, for example, by U.S. Pat. No. 4,490,444 (Isenberg). Many times the interconnection is applied by plasma spraying techniques. Such techniques can provide some leakage and lack of thermal stability. U.S. Pat. No. 5,391,440 (Kuo et al.), which also utilized a fuel electrode material coating on the interconnection, taught using a flux added mixture of LaCrO3+Cr2O3+(CaO)12.(Al2O3)7-flux- as a plasma arc spraying feed to form the interconnection, to provide leak proof, dense, stable interconnections.
In between the interconnection of one cell (or the fuel electrode material covering the interconnection of one cell) and the fuel electrode of another cell there is usually a spongy nickel felt. The nickel felt permits electrical contact between the two cells to be maintained during cell expansion and contraction which occurs as the cells are heated and cooled. Isenberg, in U.S. Pat. No. 4,648,945 found, however, that the electrical connection between the spongy nickel felt and the interconnection is sometimes poor, which increases the resistance of the cell connections and reduces the efficiency of connected cells. If fuel electrode material covers the interconnection, the poor electrical connection is between the interconnection and the fuel electrode material. Attempts to solve this problem by applying a deposit of nickel on top of the interconnection by conventional techniques, such as sputtering or plasma spraying, were not acceptable processes because they were uneconomical or introduce stresses into the cell structure.
Isenberg, in the above-described ""945 Patent, taught an elaborate process to solve the electrical connection problem involving masking the outside surface of a hollow tubular fuel cell so that only the interconnection was exposed, and then immersing a tubular hollow fuel cell in electrolyte solution containing the ions desired to deposit on the interconnection, such as nickel acetate, with a graphite bar placed inside the tube along with ammonium tartrate solution. D.C. current was then passed from the graphite bar to the outer fuel electrode anode to deposit metal on the interconnection. However, this process required manually intensive techniques to electroplate specific areas of the interconnection and necessitated that each cell/contact assembly be at least partially submerged into the electrolyte. The prior art technique deposits metal at any conductive site that is not electrically isolated from the cathode which can result in electrical shorts. In addition, extensive rinsing and cleanup are required to remove electrolyte residue. Also, variability in the physical properties relating to resistivity of the interconnection resulted in areas that did not plate or did not plate sufficiently to meet requirements. One solution was to remove the original plating deposit and reprocess the interconnection through a second electroplating. The other solution involved masking acceptably plated areas and selectively plating non-plated or areas under modified conditions to maintain the desired current density. Both solutions were more labor intensive and required consumable materials.
What is needed is a new and improved plating process for solid oxide fuel cells that can be used to electroplate the entire interconnection and/or replate specific poorly plated areas as necessary in an automated fashion.
Therefore it is a main object of this invention to provide a process to coat interconnections of solid oxide fuel cells that does not require extensive preparation, rinsing and cleanup; and does not require that solid oxide fuel cells be immersed in electrolyte.
These and other objects of the invention are accomplished by a process characterized by:
(A) providing an axially elongated tubular, hollow fuel cell comprising an outer fuel electrode, an inner air electrode and a solid electrolyte therebetween, where the electrolyte defines an elongated exposed radial segment, said segment containing an electrically conductive interconnect material;
(B) contacting the inside of the fuel cell with a cathode material without use of any liquid medium inside the fuel cell;
(C) passing electrical current through an applicator which contains liquid electrolyte solution containing a metal desired to be deposited on the interconnect material;
(D) passing electric current from the applicator, to the cathode inside of the fuel cell and contacting the interconnect with the electrolyte containing applicator and coating all of the interconnect surface with electrolyte solution so that the passage of electric current will cause metal from the electrolyte solution to coat the surface of the interconnect.
The invention also is characterized by:
(A) providing an axially elongated, tubular, hollow fuel cell comprising an outer fuel electrode, an inner air electrode and solid electrolyte therebetween, and where the electrolyte defines an elongated exposed radial segment, said segment containing a gas impermeable electrically conductive interconnect material in electrical communication with a segment of said inner air electrode; and then
(B) contacting the inside of the air electrode with a cathode material without use of any liquid medium inside the fuel cell; and then
(C) passing electrical current through an applicator which contains liquid electrolyte solution containing a metal desired to be deposited on the interconnect material; and then
(D) passing electric current from the applicator to the cathode contacting the inside of the air electrode; and then
(E) contacting a first point of the exposed interconnect with the electrolyte containing applicator and transferring the contact point to deposit electrolyte solution along the elongated axial length of the interconnect to a second point, so that the passage of electric current will cause metal from the electrolyte solution to coat the surface of the elongated axial length of the interconnect.
A plurality of fuel cell interconnects can be processed in this fashion where the applicator, preferably a rotating brush or roller, can be disposed above or below the fuel cells and movement or translation of the brushes or of the fuel cells down the axially elongated length of interconnection can be automatically programmed. This process eliminates use of liquid fluid such as a metal salt inside the fuel cell and dipping the entire exterior of the fuel cell into a container of electrolyte, thus eliminating most clean up problems and manually intensive techniques.